Dendritic cells (DC) are the professional APC of the immune system. At their immature stage, DC take up extracellular antigens by means of phagocytosis or pinocytosis and process the antigens to peptides in the endocytotic compartment such as endosomes and phagosomes, where peptides are bound to MHC class II molecules. They also have the unique ability of loading the peptides from exogenous proteins to the MHC class I pathway of presentation, a process called “cross-presentation”. Given the appropriate differentiation signals (such as microbial products), immature DC may develop into an immunogenic DC which is equipped with the ability to activate both naive and memory T cells. On the other side of the spectrum immature DC can also differentiate into a tolerogenic phenotype, which is thought to play a crucial role in the maintenance of peripheral tolerance (Steinman, Ann. Rev. Immunol. 2003, 21: 685-711; Morelli, Immunol Rev 2003: 125-146).
Numerous protocols for the generation of tolerogenic DC in vitro have been described (Xiao et al., J. Immunother. 2006 (29) 465-471). The most well-characterized methods utilise pharmacological mediators (such as immunosuppressive drugs including vitamin D3 analogues, glucocorticoids, oestrogen), cytokines and growth factors (such as IL-10, TGF-beta, IL-4 and IFN-gamma) or genetic engineering, either to suppress the expression of T cell co-stimulatory molecules (such as CD86 and CD40) or to enhance the expression of T cell inhibitory molecules (such as CTLA-4 and indoleamine 2,3-dioxygenase).
The activated form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), is a secos-teroid hormone that has, in addition to its central function in calcium and bone metabolism, important effects on the growth and differentiation of many cell types and pronounced immunoregulatory properties (van Etten et al., J Steroid Biochem and MoI Biol 2005 (97) 93-101). The biological effect of 1,25(OH)2D3 is mediated by the vitamin D receptor (VDR), a member of the superfamily of nuclear hormone receptors functioning as an agonist-activated transcription factor that binds to specific DNA sequence elements, vitamin D responsive elements, in vitamin D responsive genes and ultimately influences their rate of RNA polymerase II-mediated transcription. APC, and notably DC, express the VDR and are key targets of VDR agonists in vitro and in vivo. IL-10 is produced mainly by activated lymphocytes, monocytes and macrophages. IL-10 binds to a receptor composed of two subunits, the ligand-binding IL-10R1 and signalling IL-10R2. IL-10 down-regulates MHC class II and co-stimulatory molecule expression, IL-12 and proinflammatory cytokine secretion and T cell stimulatory function of several APC (Moore et al., Ann Rev Immunol 2001 (19)683-785).
Genetic manipulation of DC, such as inhibition of T cell co-stimulatory molecules, CD40, CD80 and CD86 by the use of antisense oligonucleotides has proven effective in generating tolerogenic DC (Machen et al., J. Immunol. 2004, 173: 4331-4341). Such DC produced reduced levels of IL-12p70 and TNF-alpha and prevented diabetes in non-obese diabetic mice.
To date, the majority of therapies approved by the US FDA for autoimmune disease have focused on the systemic inhibition of immune inflammatory activity. Although nonspecific immune suppression is partially effective in inhibiting auto-reactive immune cell function, the drugs used to suppress the immune response have numerous side effects and continuous therapy is not conductive to long-term host survival. Thus, it is desirable to develop auto-antigen-specific treatments that allow for the specific blockade of the deleterious effects of self-reactive immune cell function, while maintaining the ability of the immune system to clear infection. Hence, there is a strong need for methods that generate properly equipped DC that can efficiently induce antigen-specific immune tolerance.
In addition, ex vivo generated DC with appropriate tolerogenic function could also be implemented as therapeutic vaccine in treatment of allergy and for induction of transplant tolerance. As with immunotherapy for autoimmune diseases, efficient suppression of harmful immune responses involves the tolerance induction of both CD4+ and CD8+ T cells. Therefore, one can expect that ex vivo generated tolerogenic DC should have the same characteristics for treating autoimmune diseases, allergy and for prevention of graft rejection.
However, new and alternative methods for the production of tolerogenic dendritic cells having a distinct tolerogenic phenotype and having expression of tolerogenic determinants is always a recurring object of research in this field.